A sterilization process carried out in a sterilization chamber of a sterilizer and used to sterilize medical and hospital equipment is only effective if a certain combination of environmental conditions is achieved within the sterilization chamber of the sterilizer. For example, when steam is used as a sterilant, the object of the sterilization process is to bring steam of a suitable quality and at an appropriate temperature into contact with all surfaces of the articles being sterilized for an appropriate length of time. In some steam sterilizers the process of sterilization is typically conducted in three main phases of a sterilization cycle. In the first phase, air trapped within the article(s) being sterilized (i.e. the load being processed) is removed. The second phase is a sterilizing stage, in which the load is subjected to steam under pressure for a recognised combination of time and temperature, which is known to effect proper sterilization. The third phase is a drying phase in which condensate formed during the first two phases is removed by evacuating the chamber.
Air removal from the sterilization chamber may be achieved in a number of ways. For example, in a gravity steam sterilizer, the principle of gravity displacement is utilized, in which steam entering at the top of the chamber displaces the air which exits through a valve in the base of the chamber. In a prevacuum-type steam sterilizer, on the other hand, air is removed forcibly by deep evacuation of the chamber or by a combination of evacuation and steam injection at either subatmospheric andor superatmospheric pressures.
Any air which is not removed from the sterilization chamber during the air removal phase of the cycle or which leaks into the chamber during a subatmospheric pressure stage due to, e.g., faulty gaskets, valves or seals, may form air pockets within the load that is being sterilized. Likewise, any non-condensable gases (which, in the context of the present disclosure, means gases having a boiling point below that of the sterilant) that are present in the sterilization chamber or are carried within steam supplied to the chamber may form gas pockets within the load. These air or gas pockets will create a barrier to steam penetration, thereby preventing adequate sterilizing conditions being achieved for all surfaces of the load. This is particularly true when porous materials such as hospital linens or fabrics are being sterilized since the air or gas pockets prohibit the steam from penetrating to the interior layers of such materials.
As a result, proper sterilization may not occur. Therefore, there is a need for devices which are capable to determine the efficacy or effectiveness of sterilization cycles and, in particular, to determine the efficacy of the non-condensable gas removal stage of a sterilization cycle andor the quality of steam sterilant.
One commonly-used procedure for evaluating the effectiveness of air removal during the air removal phase of a porous load steam sterilization cycle andor for testing for the presence of non-condensable gases is known as the Bowie-Dick test. The typical Bowie-Dick test pack essentially consists of a stack of freshly laundered towels folded to a specific size, with a chemical indicator sheet placed in the centre of the stack. Chemical indicator test sheets undergo a visible change from one distinct colour to another, for example, from an initial white to a final black colour, upon exposure to the sterilization process. If the air removal within the sterilizer is insufficient, or if non-condensable gases are present during the process in sufficient quantity, an airgas pocket will form in the centre of the stack thereby preventing steam from contacting the steam sensitive chemical indicator test sheet. The consequence of inadequate steam penetration is a non-uniform colour development across the surface of the chemical indicator test sheet: thus, the presence of the airgas pocket will be recorded by the failure of the indicator to undergo the complete or uniform colour change indicative of adequate steam penetration.
Biological indicators can also be used to provide information on the effectiveness of a sterilization cycle. Parametric monitoring has also been used to either monitor or control a sterilization cycle to ensure that proper sterilization conditions are attained. For example, in U.S. Pat. No. 4,865,814 an automatic sterilizer is disclosed which includes a microprocessor which monitors both the temperature and pressure levels inside the sterilization chamber and controls a heater to allow both pressure and temperature to reach predetermined levels before starting a timer. Once the timer is started, it is stopped if the pressure or temperature levels drop below a predetermined minimum. Since it is known that the pressure and temperature variables of saturated steam are mutually dependent variables when saturated steam is enclosed in a sealed chamber, monitoring of these two variables can ensure that proper conditions are maintained during the sterilization cycle.
Although it is desirable to monitor environmental conditions within the sterilization chamber itself, it is generally considered more desirable to be able to monitor the environmental conditions within an actual load being sterilized or within a test pack (such as the Bowie-Dick test pack) that represents such a load. However, the typical Bowie-Dick test pack presents many disadvantages. Since the Bowie-Dick test pack is not preassembled, it must be constructed every time the procedure is used to monitor sterilizer performance. The preparation, assembly and use of the Bowie-Dick test pack is time consuming and cumbersome and, moreover, varying factors, such as laundering, pre-humidification, towel thickness and wear, and the number of towels used, alter the test results.
Therefore, alternative sterilizer testing systems have been developed to overcome these limitations. For example, WO 9712637 describes a sterilant challenge device for use in a sterilizer for determining the efficiency of the air removal stage of a sterilization cycle. The device comprises a tube of thermally-insulating material, the bore of the tube defining a free space which is open at one end for the entry of sterilant and is closed at the other end; a plurality of thermally-conductive masses located around the tube, along the length of the latter, the masses being thermally-separated from one another; and a thermal insulation surrounding the tube and the thermally-conductive masses, whereby the penetration of sterilant along the bore of the tube during a sterilization cycle is inhibited through the accumulation of air andor non-condensable gas within the free space resulting from the condensation of moisture on the walls of the bore.
The thermally-insulating material suggested by WO 97/12637 is a tube made of Liquid Crystal Polymer (LCP), preferably of a complete aromatic copolyester with a 25% by weight graphite content. Despite the fact that such LCP tubes can be desirably optimized for commercial use, they are not perfect. For example, since the LCP tube is typically die cast it exhibits an anisotropic thermal conductivity: the thermal conductivity along the longitudinal axis of the LCP tube is larger than along a radial direction. This may distort the results since the temperature at a given location of the tube does thus not only depend on the steam temperature at said location but also on the heat transport from a longitudinally displaced location along the tube to said location. Moreover, LCP is more difficult to handle than other materials. In particular, with LCP it is extremely difficult to reliably provide a high level of reproducibility of the test results. Finally, the rates of expansioncontraction of LCP on the one hand and of the thermally conductive material of the thermally conductive masses on the other hand are quite different. Thus, a relatively complex design of the thermally conductive masses is needed in order to accommodate these different rates of expansioncontraction. Typically, said design consists of two halves being held together by two spring clips.